The present invention relates to corrosion of components in various settings. More specifically, the present invention relates to monitoring such corrosion.
Corrosion is the gradual destruction of materials by chemical or other reaction with its environment. Corrosion degrades the useful properties of materials and structures including strength, appearance, and permeability to fluids. Many structural alloys corrode merely from exposure to moisture in air but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area uniformly corroding the surface.
The field of corrosion measurement, control, and prevention is very broad. Corrosion measurement employs a variety of techniques to determine how corrosive the environment is and at what rate metal loss is being experienced. Some corrosion measurement techniques can be used on-line, constantly exposed to the process stream, while others provide off-line measurement, determined in a laboratory analysis. Some techniques give a direct measure of metal loss or corrosion rate while others are used to infer that a corrosive environment may exist.
The rate of corrosion dictates how long any process plant can be usefully and safely operated. The measurement of corrosion and the action to remedy high corrosion rates permits the most cost effective plant operation to be achieved while reducing the life-cycle costs associated with the operation.
The following list details common corrosion monitoring techniques which are used in industrial applications.                Corrosion Coupons (weight loss measurements)        Electrical Resistance (ER)        Linear Polarization Resistance (LPR)        Galvanic (ZRA) I Potential        Hydrogen Penetration        Microbial        Sand/Erosion        
The weight loss technique is the best known and simplest corrosion monitoring technique. The method involves exposing a specimen of material (coupon) to a process environment for a given duration, then removing the specimen for analysis. The basic measurement which is determined from corrosion coupons is weight loss. Corrosion rate can be calculated by dividing the weight loss by material density, coupon surface area, and time of exposure. Coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques.
ER probes can be thought of as “electronic” corrosion coupons. ER probes provide a basic measurement of metal loss and the value of metal loss can be measured at any time while the probe is in-situ. The ER technique measures the change in electrical resistance of a corroding metal element exposed to the process. The action of corrosion on the surface of the element produces a decrease in its cross-sectional area with a corresponding increase in its electrical resistance.
The LPR technique is based on electro-chemical theory. A small voltage is applied to an electrode in solution. The current needed to maintain a specific voltage shift (typically 10 mV) is directly related to the corrosion on the surface of the electrode in the solution. By measuring the current a corrosion rate can be derived. The advantage of the LPR technique is that the measurement of corrosion rate is made instantaneously whereas with coupons or ER some period of exposure is required to determine corrosion rate. The LPR technique can only be performed in clean aqueous electrolytic environments and will not work in gases.
Corrosion is an expense in many systems. The impact of corrosion includes lost production, system down time, system failures, as well as repair time and expense. There is an important need to prevent and monitor corrosion.